1. Field of the Invention
This invention relates generally to roofing materials and in particular to materials for level flat roofs of office buildings, shopping centers, industrial buildings and the like. Still more particularly the invention relates to a composite sheet material and its method of manufacture which is essentially waterproof, relatively insensitive to ultraviolet light, and easily applied as a roof covering to a flat level surface.
2. Description of the Prior Art
Sheet materials containing elastomeric or rubber compounds have been long used in the art for providing a roofing layer which is essentially waterproof. Recently, a thermoplastic rubber product, sold under the registered trademark SANTOPRENE by the Monsanto Company, has been extruded into a sheet material suitable for use as a roof covering.
The SANTOPRENE material is essentially a composition of cured or vulcanized ethylene propylene diene monomer rubber in a matrix of polypropylene. The percentages of components vary depending on the particular grade of SANTOPRENE used. It may be described as exhibiting the elastomeric performance of vulcanized rubbers and the processing simplicity of thermoplastic polymers.
The SANTOPRENE material is supplied as small free flowing pellets which may be extruded in a sheet die to form sheet material. Special additives have been added to the SANTOPRENE material in the past to increase its ultraviolet radiation resistance. Other additives serve to provide flame resistance to the SANTOPRENE material.
While the SANTOPRENE product results in a marginally acceptable roofing material when extruded into sheet form, its relatively high cost makes it difficult to compete in price with other rubberized roofing sheet materials. In addition, better performance of a composite sheet material has been desired than heretofore have been achievable with single layer SANTOPRENE.